1. Field of the Invention
The present invention relates to a method and apparatus for delayed optical logic gates, where the physics lies in quantum-coherence-based slow light and enhanced nondegenerate four-wave mixing processes.
2. Description of the Related Art
In an electronic transistor, the switching time is determined by the carriers' transfer time across a gate, where the size of the transistor gate has been decreased for last three decades. This is called Moore's law and has been succeeded. Nowadays silicon semiconductor technologies are migrating into photonics, and this trend is called silicon photonics. In the fiber-optic communications, most optical devices are controlled by electric current or electric voltage, where the control circuit is Si-based electronics. However the electronics is much slower than optics. Thus speed constraint of an electrooptic device lies in the electronics. Recently, quantum switch and photon logic gates are introduced to overcome such limitations in the switching technologies: B. S. Ham, U.S. Pat. No. 6,628,453 (2003); B. S. Ham, PCT patent filing number PCT/KR2007/001130 (2007).
On the other hand, in fiber-optic communications, optical switching technologies have been rapidly evolved for faster switching components. In this optoelectronics area electronic counterpart is lagged behind in speed itself. Thus overall performance of all-optical switching devices is should be slowed down. Obviously computer-based electro-optic device has the speed constraint on the electronics.
To overcome such unbalance between optics and electronics, buffering techniques have been introduced. This buffering method is to delay the fast optical data traffic on demand for the relatively slow electronic components. Thus, adjustable buffering memory techniques are expected. Unfortunately all-optical active buffer memory has not been implemented, yet. Most of the suggested ideas, inventions or devices, however, are passive like such as using a bundle of fiber-optic cables and a ring resonator on a silicon waveguide.
It is well known that resonant two-color electromagnetic fields can induce a refractive index change in a nonlinear optical medium composed of three energy levels or more. In a three-level optical system composed of two-closely spaced ground states, the refractive index change can result in not only absorption cancellation at line center but also two-photon coherence excitation on the ground levels. This phenomenon is called dark resonance or electromagnetically induced transparency (EIT) in the context of optically dense medium: S. E. Harris, Phys. Today. Vol. 50 (No. 7), p. 36 (1997); Phys. Rev. Lett. Vol. 62, pp. 1033-1036 (1989)). Because EIT modifies the absorption spectrum of an optical medium, the medium's dispersion must be also modified via Kramers Kronig relations. The modified dispersion profile directly affects on a group velocity of a traveling light pulse through the optical medium. This is so called a slow light phenomenon. Recently the slow light phenomenon has been observed in cold atoms (Hau et al., Nature Vol. 397, pp. 594-598 (1999)), defected solids (B. S. Ham et al., Phys. Rev. Lett. Vol. 88, p. 236024 (2002), s fiber-coupled resonator (Totsuka et al., Phys. Rev. Lett. Vol. 98, p. 213904 (2007), and semiconductors (Wang et al., Opt. Lett. Vol. 29, pp. 2291-2293 (2004).
An optical router is a switching device converting an optical signal into another one at different propagation directions with either the same frequency basis or not. The optical router is a subcategory of an optical switch that is in general used to drop, add, multiplex, or convert an optical signal into another one. In fiber-optic communications, as more data traffic is demanded, more information bandwidth is needed. In this case a wider bandwidth optical switch is obviously expected. So far optical switching speed is already passed over 60 GHz, which is much faster than a Pentium CPU. Thus, more often it is required that the data traffic in fiber-optic communication lines need to be temporally delayed for some data processing purposes. Obviously, an optical buffer memory becomes an essential component to an optical data processing unit.
A delayed optical router has been suggested. According to the delayed optical router, a slow light is used for routing via nondegenerate four-wave mixing processes. Thus the delayed optical router can delay the input data up to the group delay time, which is the delay time can be controllable by adjusting coupling light intensity C in FIG. 1: Ham et al., Physical Review Letters, Vol. 88, p. 236024 (2002).